The interdiction of contraband radioactive material, illicit drugs, explosives, and other contraband is an important goal of law enforcement. To that end, a variety of technologies have been developed and deployed for the non-intrusive inspection of containers not readily susceptible to visual scrutiny from the outside. The non-intrusive aspect of these inspection techniques is important; the great majority of containers do not carry contraband, and the public would not long tolerate the delays, disruption (and in some cases damage) of property, and invasions of privacy that would occur if invasive inspection means were commonly used. Non-intrusive inspection is typically non-destructive and can usually be accomplished faster than intrusive inspection, thereby increasing productivity of inspectors. Increased productivity means more containers inspected and more contraband interdicted.
The present invention is primarily concerned with the non-intrusive inspection of containers for the presence of contraband radioactive material. Such material could be used to create nuclear bombs or radiological dispersal devices (RDD)—known as “dirty bombs,” to contaminate food or water supplies, or potentially to further the cause of terrorism.
Contraband radioactive material could be smuggled in nearly any size container including, for example, luggage, or large cargo containers. Only a small volume of such material would be required to make a device capable of large-scale destruction or contamination. In light of this problem, a large number of containers of varying sizes would have to be monitored for the presence of contraband radioactivity in order to prevent a potential nuclear catastrophe. Accurate detection is of paramount importance due to the potentially disastrous consequences of permitting even a single container with contraband radioactivity past a monitored checkpoint. Additionally, due to the number of containers that would have to be surveyed, such surveying would have to be as fast and efficient as possible to be of any real utility in practical applications. Slow survey times would impose unbearable burdens of commerce and the flow of traffic through checkpoints.
One current method for screening containers for the presence of radioactive isotopes is to blindly survey external surfaces of the container, using an emission detector. The method is demonstrated by U.S. Pat. No. 4,291,227 (Caldwell, et al). A problem with this method is that sensitivity for detection varies in an unknown manner, over orders of magnitude, depending on the location and effectiveness of any shielding within the container. Consequently, any survey conducted within any reasonable time will likely fail to detect shielded sources. Additionally, in order to perform a blind external survey that would locate shielded contraband radioactive material would be prohibitively time-consuming if the maximum shielding is assumed for safety purposes, yet without knowledge of the potential effect of shielding within the container, it is not possible to estimate how much radioactive material may escape detection.
An additional drawback of the system is that if detectors with large field of view and large surface areas are used, the background radioactivity produced by the contents of the container is also included within the field. Well known sources of background that impair monitoring include natural radioactivity of the constituent materials of the detector itself, and of nearby equipment, supports, and shielding, terrestrial radiation, construction materials, radioactivity in the air surrounding the detector, and the primary and secondary components of cosmic radiation. Electronic noise of the instrument itself may also be apparent as background. It is desirable that the ratio of the radioactivity of interest to the background be as high as possible, to favor detection. Large field of views detectors tend to capture more background radioactivity, so the target-to-background ratio, an index of the signal to noise for detection, is degraded. The quantity and distribution of background radiation in a container may vary, and blind emission scanning may be insensitive to the quantity and location of these variations within the container. A special case of emission imaging to detect contraband materials, including special nuclear materials (SNM) or explosives and other contraband is neutron activation Neutron activation is commonly performed blindly, without quantification of the effect of shielding materials in the cargo on the sensitivity of the method.
Another method for the detection of contraband radioactive material is strictly through the use of transmission imaging (typically, digital planar or computed tomographic X-rays). An example of this method, using high-powered X-ray transmission imaging is U.S. Pat. No. 6,347,132 (Annis). This method requires high-energy X-rays in a fixed installation or other shielded environment. Additionally, there is no provision for differentiating between high-density non-radioactive elements which are not gamma-emitters. This method, in common with other transmission image methods such as gamma rays, considered equivalent to a high energy X-ray, is also used to show objects that could possibly comprise shielding, or in some cases, radioactive materials themselves if high in density. Transmission imaging provides this and other valuable information, but is not satisfactory for detecting contraband radioactive sources when used alone for several reasons. Limitations of transmission imaging alone include: skilled personnel are required to interpret the images that are produced to determine whether or not further (intrusive) analysis is required. Image interpretation is flawed in that it is based on subjective criteria and, thus, detection may be readily avoided. Second, radioactive sources or shielding do not have any specific appearance, and many can easily escape detection (i.e., by appearing to be an otherwise innocent object): An important instance, liquid shielding, will typically elude this method, since they appear dense but otherwise innocent on transmission images. It is not apparent by inspection alone the quantity of radioactivity that shielding materials may conceal.
Thus, it is clear that the current methods of detecting contraband radioactivity are individually flawed in that they either will miss significant sources or will require excessive time to perform adequate surveys will be unacceptable to efficiently handle the volume of containers that would require analysis, require skilled observers, do not allow specification in advance of the sensitivity of the detection system, and do not allow efficient reduction of the effects of background activity.